PROJECT SUMMARY Over the years it has become evident that the immune system can affect the activity of the central nervous system (CNS), including altering cognitive processes. The impact of immune activation on the CNS is particularly important for aged individuals, as the brain's resident immune cells, microglia, acquire a pro- inflammatory profile. Understanding why these cells show increased inflammatory activity (i.e., neuroinflammation) and identifying effective treatments to reduce microglia activation is expected to have beneficial effects on cognitive performance and measures of neural plasticity. The long-term goal of this research program is to identify the physiological underpinnings and functional consequences of increased neuroinflammation in aged subjects and evaluate interventions to delay or prevent these changes. The objective of this application is to identify means of regulating microglia proliferation and assess the contribution of age-related increases in microglia activity to cognitive function. The central hypothesis is that increased microglia proliferation contributes to the heightened neuroinflammation and cognitive decline associated with aging and that increasing physical activity will reduce neuroinflammation. Guided by our preliminary data, this hypothesis will be tested by completion of three specific aims: 1) Identify mechanisms of age-related increases in microglia proliferation. 2) Determine the extent to which increased microglia proliferation in aged subjects contributes to reduced hippocampal neurogenesis, cognitive deficits, and neuroinflammation. 3) Determine whether exercise attenuates proliferation and expression of the proinflammatory phenotype in microglia. The first aim will employ pharmacological manipulations to increase or decrease two endogenous stimulants of microglia proliferation that were found to be increased in aged subjects. Triple-label immunofluorescence will be used to quantify microglia proliferation. In aim 2, spatial learning ability, hippocampal neurogenesis, and neuroinflammation will be evaluated following pharmacological inhibition of microglia proliferation and in P2X7 receptor knockout mice. Our approach for aim 3 is to assess the impact of exercise on the proliferation and activation state of microglia, via flow cytometry and immunofluorescence, in young and aged mice. The present application is innovative, in our opinion, because it focuses on a novel approach to reduce neuroinflammation, namely, through regulating microglia proliferation. Further, it merges the fields of neurobiology and immunology, to understand age-related changes in cognition and neural plasticity. This contribution is significant because it is expected to facilitate development of novel pharmaceutical treatments and explores non-drug based therapies in the form of physical exercise to attenuate chronic neuroinflammation. Ultimately, such knowledge has the potential to facilitate development of efficacious treatments for attenuating neuroinflammation that in the context of aging will help reduce the growing problem of age-related cognitive decline and neuropathologies.